(1) Field of the Invention
The present invention relates to a method for manufacturing a cam-cam shaft assembly in which a cam piece made of a specified wear-resistant sintered alloy is jointed to a cam shaft of a steel pipe or the like.
(2) Background of the Invention
As the cam-cam shaft assembly for use in the internal combustion engines, ones in which a cam and a cam shaft are integrally cast commonly using a ordinary cast iron, or chilled cast casting of an alloyed cast iron have been employed. Further, recently taking into account the improvement in engine performance, reduction in weight and cost, and so forth, a special sintered alloy has beem employed as a cam portion of a sliding face. A cam-cam shaft assembly which is constituted by assembling a component like a cam piece, a journal piece or the like onto a pipe-like steel shaft or the like has been proposed.
However, the cam shaft and the cam in the conventional cam-cam shaft assemblies have frequently been joined together by means of a secondary method such as brazing, welding, mechanical caulking or the like. Such conventional methods inevitably require a special machine or device for this purpose. In addition, since the number of components to be assembled onto the shaft is relatively large, troublesome joining steps can not be avoided.
For the purpose of eliminating such defects encountered in the conventional cam-cam shaft assemblies, there have been proposed the diffusion joining method in which a cam, a journal piece or the like which is made of a specified sintered alloy producing a liquid phase during sintering is metallurgically joined to a steel pipe shaft. According to this method, a component, such as a cam piece, made of the above specified alloy is presintered, and this presintered component is assembled onto the shaft by press-in method or by clearance-caulking method; and then the presintered component as assembled is sintered under a predetermined conditions to change it to a wear-resistant component such as a cam and at the same time effect the metallic bonding of the presintered component to the shaft member. Therefore, the method as just mentioned above is advantageous from the standpoint of the simplification of the manufacturing procedures, the cost reduction and so forth.
However, since many of the sintered alloys contracts or expands when heated during the sintering step, the inner diameter of the component such as a cam piece become larger and therefore, a firm and rigid joint between the component and the shaft can not necessarily be obtained. In addition to the requirement for the firm joining, such alloy for the component further requires wear resistance and therefore, a specified sintered alloy had to be discovered for this purpose.
In accordance with another method as previously proposed, there is employed a wear resistant iron-base sintered alloy comprising iron containing appropriate proportions of carbon, molybdenum, phosphorus, boron, and optionally copper and/or cobalt etc. which is capable of producing a liquid phase during sintering. However, the presintered cam piece made of such a wear resistant iron-base alloy as assembled onto the shaft is so joined to the shaft that during the course of sintering, the inner diameter of the cam piece expands by as much as slightly larger than 1% thereof and finally contracts as much as slightly larger than 1%. Thus, there remain the problems that the locating of the sintered component onto the pipe shaft is not necessarily attained with accuracy and that the joining strength becomes insufficient due to a small amount of the contraction. Therefore, reliability of the joining is not necessary satisfactory.
In case that the assembling interference, that is, the inner diameter of the cam piece with respect to the outer diameter of the cam shaft is designed smaller so as to compensate a small amount of the contraction, no expected effects can be obtained during the press-in procedure and/or during the succeeding procedures because the presintered mass has a small mechanical strength.